Identifying solar-like magnetic cycles with Zeeman-Doppler-Imaging (ZDI)

TL;DR

This study uses simulations and Zeeman-Doppler-Imaging to identify optimal magnetic field parameters that reveal solar-like magnetic cycles, highlighting the axisymmetric energy fraction as a key observable.

Contribution

It demonstrates that the axisymmetric energy fraction from ZDI observations effectively traces solar-like magnetic cycles, providing a new method to analyze stellar magnetic activity.

Findings

Axisymmetric energy fraction best traces solar-like cycles

ZDI can recover the increase of toroidal energy with S-index

ZDI may reveal dynamo modes and small-scale flux emergence properties

Abstract

We are reaching the point where spectropolarimetric surveys have run for long enough to reveal solar-like magnetic activity cycles. In this paper we investigate what would be the best strategy to identify solar-like magnetic cycles and ask which large-scale magnetic field parameters best follow a solar-type magnetic cycle and are observable with the Zeeman-Doppler-Imaging (ZDI) technique. We approach these questions using the 3D non-potential flux transport simulations of \cite{Yeates2012} modelling the solar vector magnetic field over 15 years (centred on solar cycle 23). The flux emergence profile was extracted from solar synoptic maps and used as input for a photospheric flux transport model in combination with a non-potential coronal evolution model. We synthesise spectropolarimetric data from the simulated maps and reconstruct them using ZDI. The ZDI observed solar cycle is set into the context of other cool star observations and we present observable trends of the magnetic field topology with time, sunspot number and S-index. We find that the axisymmetric energy fraction is the best parameter of the ZDI detectable large-scale field to trace solar-like cycles. Neither the surface averaged large-scale field or the total magnetic energy is appropriate. ZDI seems also to be able to recover the increase of the toroidal energy with S-index. We see further that ZDI might unveil hints of the dynamo modes that are operating and of the global properties of the small-scale flux emergence like active latitudes.